A type of communication in which e a wireless terminal directly communicates with another wireless terminal without going through an infrastructure network such as a base station is referred to as device-to-device (D2D) communication. The D2D communication includes at least one of Direct Communication and Direct Discovery. In some implementations, a plurality of wireless terminals supporting D2D communication form a D2D communication group autonomously or under the control of a network, and communicate with another wireless terminal in the formed D2D communication group.
Proximity-based services (ProSe) specified in 3GPP Release 12 is one example of the D2D communication (see, for example, Non Patent Literature 1). ProSe Direct Discovery is performed through a procedure in which a wireless terminal capable of performing ProSe (i.e., ProSe-enabled User Equipment (UE)) discovers another ProSe-enabled UE only by using the capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) of those two UEs. ProSe Direct Discovery may be performed by three or more ProSe-enabled UEs.
ProSe Direct Communication makes it possible to establish a communication path(s) between two or more ProSe-enabled UEs existing in a direct communication range after the ProSe Direct Discovery procedure is performed. Stated differently, ProSe Direct Communication enables a ProSe-enabled UE to directly communicate with another ProSe-enabled UE without going through a Public Land Mobile Network (PLMN)) including a base station (eNodeB (eNB)). ProSe Direct Communication may be performed by using a radio communication technology (i.e., E-UTRA technology) that is also used to access a base station (eNB) or by using a Wireless Local Area Network (WLAN) radio technology (i.e., IEEE 802.11 radio technology).
In 3GPP Release 12, a radio link between wireless terminals used for Direct Communication or Direct Discovery is referred to as Sidelink (see, for example, Section 14 in Non Patent Literature 2). Sidelink transmission uses the Long Term Evolution (LTE) frame structure defined for uplink and downlink and uses a subset of uplink resources in frequency and time domains. A wireless terminal (i.e., UE) performs sidelink transmission by using Single Carrier FDMA (Frequency Division Multiple Access) (SC-FDMA), which is the same as used in uplink.
In 3GPP Release 12 ProSe, allocation of radio resources to a UE for sidelink transmission is performed by a radio access network (e.g., Evolved Universal Terrestrial Radio Access Network (E-UTRAN)). A UE that has been permitted to perform sidelink communication by a ProSe function performs ProSe Direct Discovery or ProSe Direct Communication by using radio resources allocated by a radio access network node (e.g., eNB (eNB)).
As for ProSe Direct Communication, two resource allocation modes, i.e., scheduled resource allocation and autonomous resource selection, are defined. The scheduled resource allocation and the autonomous resource selection are referred to as “sidelink transmission mode 1” and “sidelink transmission mode 2”, respectively (see Section 14 in Non Patent Literature 2).
In the scheduled resource allocation for ProSe Direct Communication, when a UE desires to perform sidelink transmission, this UE requests an eNB to allocate radio resources for sidelink transmission, and the eNB allocates resources for sidelink control and data to the UE. To be specific, a UE transmits to an eNB a scheduling request to request an uplink (UL) data transmission resource (i.e., Uplink Shared Channel (UL-SCH) resource) and then transmits a Sidelink Buffer Status Report (Sidelink BSR) to the eNB by using an UL data transmission resource allocated by an uplink grant (UL grant). The eNB determines sidelink transmission resources to be allocated to the UE based on the Sidelink BSR and transmits a sidelink grant (SL grant) to the UE.
The SL grant is defined as Downlink Control Information (DCI) format 5. The SL grant (i.e., DCI format 5) contains contents such as a Resource for PSCCH, Resource block assignment and hopping allocation, and a time resource pattern index. The Resource for PSCCH indicates radio resources for a sidelink control channel (i.e., Physical Sidelink Control Channel (PSCCH)). The Resource block assignment and hopping allocation is used to determine frequency resources, i.e., a set of subcarriers (resource blocks), for transmitting a sidelink data channel (i.e., Physical Sidelink Shared Channel (PSSCH)) for sidelink data transmission. The Time resource pattern index is used to determine time resources, i.e., a set of subframes, for transmitting the PSSCH. Note that, strictly speaking, the resource block means time-frequency resources in LTE and LTE-Advanced and is a unit of resources specified by consecutive OFDM (or SC-FDMA) symbols in the time domain and consecutive subcarriers in the frequency domain. In the case of Normal cyclic prefix, one resource block includes 12 consecutive OFDM (or SC-FDMA) symbols in the time domain and 12 subcarriers in the frequency domain. That is, the Resource block assignment and hopping allocation and the Time resource pattern index designate a resource block for transmitting the PSSCH. The UE (i.e., a sidelink transmitting terminal) determines a PSCCH resource and a PSSCH resource according to the SL grant.
On the other hand, in the autonomous resource selection for ProSe Direct Communication, a UE autonomously selects resources for sidelink control (i.e., PSCCH) and data (i.e., PSSCH) from a resource pool(s) set by an eNB. The eNB may allocate a resource pool(s) for the autonomous resource selection to the UE in a System Information Block (SIB) 18. The eNB may allocate a resource pool for the autonomous resource selection to the UE in Radio Resource Control (RRC)_CONNECTED by dedicated RRC signaling. This resource pool may be usable also when the UE is in RRC IDLE.
When direct transmission is performed on a sidelink, a UE on a transmitting side (i.e., a D2D transmitting UE) (hereinafter referred to as a transmitting terminal) transmits Scheduling Assignment information by using a portion of radio resources (i.e., resource pool) for a sidelink control channel (i.e., PSCCH). The scheduling assignment information is also referred to as Sidelink Control Information (SCI) format 0. The scheduling assignment information includes contents such as resource block assignment and hopping allocation, a time resource pattern index, and a Modulation and Coding Scheme (MCS). In the case of the above-described scheduled resource allocation, the Resource block assignment and hopping allocation and the time resource pattern index indicated by the Scheduling Assignment (i.e., SCI format 0) conform to the Resource block assignment and hopping allocation and the time resource pattern index indicated by the SL grant (i.e., DCI format 5) received from the eNB.
The transmitting terminal transmits data on the PSSCH by using a radio resource according to the scheduling assignment information. A UE on a receiving side (i.e., a D2D receiving UE) (hereinafter referred to as a receiving terminal) receives the scheduling assignment information from the transmitting terminal on the PSCCH and receives the data on the PSSCH according to the received scheduling assignment information. Note that, the term “transmitting terminal” just focuses on a transmission operation of a wireless terminal and does not mean a radio terminal dedicated for transmission. Similarly, the term “receiving terminal” is an expression for expressing a receiving operation of a wireless terminal and does not mean a wireless terminal dedicated for reception. That is, the transmitting terminal is able to perform a receiving operation and the receiving terminal is able to perform a transmitting operation.
Hereinafter, a sidelink control period, a resource pool for PSCCH and a resource pool for PSSCH are described. These are required to determine radio resources (i.e., subframes and resource blocks) for transmitting a PSCCH and radio resources for transmitting a PSSCH. As described earlier, the PSCCH is a sidelink physical channel to be used for transmission of sidelink control information (SCI) such as scheduling assignment information. On the other hand, the PSSCH is a sidelink physical channel to be used for user data transmission (direct transmission).
The sidelink control period is a scheduling period for sidelink (see FIG. 1). The sidelink control period is also referred to as a PSCCH period. The transmitting terminal transmits scheduling assignment information (i.e., SCI format 0) in each sidelink control period. In 3GPP Release 12, the sidelink control period is 40 milliseconds (ms), 60 ms, 70 ms, 80 ms, 120 ms, 140 ms, 160 ms, 240 ms, 280 ms or 320 ms. In other words, the sidelink control period is 40 subframes, 60 subframes, 70 subframes, 80 subframes, 120 subframes, 140 subframes, 160 subframes, 240 subframes, 280 subframes or 320 subframes.
Therefore, the transmitting terminal notifies the receiving terminal of the allocation of PSSCH resources in each sidelink control period, i.e., every 40 ms or more. Note that, however, the allocation of PSSCH resources is specified in units of 6, 7 or 8 subframes (6, 7 or 8 ms) by use of the time resource pattern index. Thus, in one sidelink control period, the same PSSCH resource allocation is used periodically with a period of 6, 7 or 8 subframes.
In one sidelink control period, the transmitting terminal transmits scheduling assignment information (i.e., SCI format 0) two times in two subframes out of LPSCCH number of subframes contained in a resource pool (subframe pool) for PSCCH. The two times of transmission is performed in two different resource blocks among mPSCCH_RPRB number of resource blocks contained in a resource pool (resource block pool) for PSCCH.
The resource pool for PSCCH is set to a UE by an eNB via broadcasting (SIB 18) or dedicated RRC signaling. The resource pool for PSCCH consists of LPSCCH number of subframes and MPSCCH_RPRB number of frequency domain resource blocks in a sidelink control period.
A method for specifying a resource pool for PSCCH is described hereinafter with reference to FIGS. 2 and 3. A PSCCH resource pool consists of a subframe pool and a resource block pool. FIG. 2 shows a PSCCH subframe pool, and FIG. 3 shows a resource block pool for PSCCH.
An eNB specifies a length (P) of the sidelink control period (PSCCH period), and a PSCCH subframe bitmap and its length (N′) in order to identify the PSCCH subframe pool. The length (N′) of the subframe bitmap is 4, 8, 12, 16, 30, 40 or 42 bits. The N′ subframes corresponding to the subframe bitmap are the first N′ subframes within the sidelink control period as shown in FIG. 2. The subframe bitmap indicates that a subframe corresponding to a bit that is set to “0” is not used for PSCCH transmission and a subframe corresponding to a bit that is set to “1” can be used for PSCCH transmission. Accordingly, the number of subframes (LPSCCH) contained in the PSCCH resource pool in one sidelink control period is equal to the number of bits that are set to “1” within the subframe bitmap. The subframes contained in the PSCCH resource pool (i.e., subframe pool) can be represented as follows:(l0PSCCH,l1PSCCH, . . . ,lLPSCCH−1PSCCH).  [Expression 1]
On the other hand, as shown in FIG. 3, the eNB specifies the index (S1) of a start Physical Resource Block (PRB), the index (S2) of an end PRB, and the number of PRBs (M) in order to identify a resource block pool for PSCCH. The resource block pool contains M number of PRBs the PRB index q of each of which is equal to or more than the start index (S1) and less than S1+M (i.e., S1<=q<S1+M) and M number of PRBs the PRB index q of each of which is more than S2−M and equal to or less than the end index (S2) (i.e., S2−M<q<=S2), i.e., the total number of PRBs is 2M. Thus, the eNB can include two PRB clusters, each containing M number of PRBs, into the resource block pool for PSCCH.
A method for specifying a resource pool for PSSCH is described hereinafter. In the case of the scheduled resource allocation (i.e., sidelink transmission mode 1), the eNB specifies a PSSCH subframe pool via SIB 18 or dedicated signaling (RRC signaling). The sidelink control period (PSCCH period) that is associated with the PSCCH resource configuration is also associated with the PSSCH resource configuration. The UE determines the PSSCH resource pool consisting of a subframe pool as follows. Specifically, as shown in FIG. 2, in the sidelink control period (PSCCH period), subframes each having the subframe index equal to or more than lPSCCHPSCCH-+1 belong to the PSSCH subframe pool.
On the other hand, in the case of the autonomous resource selection (i.e., sidelink transmission mode 2), the eNB specifies a PSSCH subframe pool and a resource block pool via SIB 18 or dedicated signaling (RRC signaling). The eNB specifies an offset (O2), a subframe bitmap and its length (NB) in order to specify the subframe pool.
The offset (O2) indicates an offset from the subframe index jbegin of the first subframe in the sidelink control period (i.e., PSCCH period). In this example, it is assumed that the number of subframes each having the subframe index equal to or more than jbeginO2 in the PSCCH period is N′.
The length (NB) of the subframe bitmap is 4, 8, 12, 16, 30, 40 or 42 bits. The subframe bitmap indicates that a subframe corresponding to a bit that is set to “0” is not used for PSSCH transmission and a subframe corresponding to a bit that is set to “1” can be used for PSSCH transmission. Note that, in normal cases, the length (NB) of the subframe bitmap is smaller than the total number (N′) of subframes each having the subframe index equal to or more than jbegin+O2 in the PSCCH period. Thus, the UE determines a bitmap b0, b1, b2, . . . , bN′-1 using the following equation:bj=aj mod NB, for 0≤j<N′,  [Expression 2]where a0, a1, a2, aN_B-1 is the bitmap with the length NB that is indicated in the PSSCH configuration by the eNB. If bj=1, a subframe lj belongs to the PSSCH subframe pool.
The resource block pool for PSSCH in the case of the autonomous resource selection (sidelink transmission mode 2) is specified in the same manner as the resource block pool for PSCCH. Specifically, in order to identify the resource block pool for PSSCH, the eNB specifies the index (S1) of a start Physical Resource Block (PRB), the index (S2) of an end PRB, and the number of PRBs (M) by the PSSCH resource configuration.